* Send the invoice to the client.
    * Collect the money from the client.
    * Send a notification back to the API user (the external developer), as a callback.
        * At this point is that the API will somehow send a POST request to the
            external API with the notification of the invoice event
            (e.g. "payment successful").
    """
    # Send the invoice, collect the money, send the notification (the callback)

    /*
     * Since we use AtomicReferenceFieldUpdaterAtomicHelper by default, we'll "obviously" use it
     * even when Unsafe isn't available. But it's nice to have a check here to make sure that
     * nothing somehow goes wrong as the JDK restricts access to Unsafe.
     */
    checkHelperVersion(NO_UNSAFE, "AtomicReferenceFieldUpdaterAtomicHelper");
    /*

 *     classes can be chained onto them without casting.
 * @param <G> The type of the generator to be passed to testers in the generated test suite. An
 *     instance of G should somehow provide an instance of the class under test, plus any other
 *     information required to parameterize the test.
 * @author George van den Driessche
 */
@GwtIncompatible
public abstract class FeatureSpecificTestSuiteBuilder<

 *     classes can be chained onto them without casting.
 * @param <G> The type of the generator to be passed to testers in the generated test suite. An
 *     instance of G should somehow provide an instance of the class under test, plus any other
 *     information required to parameterize the test.
 * @author George van den Driessche
 */
@GwtIncompatible
public abstract class FeatureSpecificTestSuiteBuilder<

* etc.

In these cases, you would normally return an **HTTP status code** in the range of **400** (from 400 to 499).

This is similar to the 200 HTTP status codes (from 200 to 299). Those "200" status codes mean that somehow there was a "success" in the request.

The status codes in the 400 range mean that there was an error from the client.

Remember all those **"404 Not Found"** errors (and jokes)?

      TestCondition testCondition = create();

      ScheduledExecutorService scheduledPool = newScheduledThreadPool(1);
      // If signal() fails somehow, we should see a failed test, even without looking at the Future.
      Future<?> unused = scheduledPool.schedule(testCondition::signal, delay, unit);

      return testCondition;
    }

```Python hl_lines="2"
@app.get('/')
def results():
    results = some_library()
    return results
```

---

If your application (somehow) doesn't have to communicate with anything else and wait for it to respond, use `async def`, even if you don't need to use `await` inside.

---

If you just don't know, use normal `def`.

---

      TestCondition testCondition = create();

      ScheduledExecutorService scheduledPool = newScheduledThreadPool(1);
      // If signal() fails somehow, we should see a failed test, even without looking at the Future.
      Future<?> unused = scheduledPool.schedule(testCondition::signal, delay, unit);

      return testCondition;
    }

  }

  /**
   * Returns an immutable map containing the same entries as {@code map}. The returned map iterates
   * over entries in the same order as the {@code entrySet} of the original map. If {@code map}
   * somehow contains entries with duplicate keys (for example, if it is a {@code SortedMap} whose
   * comparator is not <i>consistent with equals</i>), the results of this method are undefined.
   *